The edge roughness of test patterns fabricated by ion projection lithography has been investigated to indicate the optimum experimental conditions for sub-100-nm resolution. Direct observation in a scanning electron microscope, electron metrology, and atomic force microscopy has been employed for the evaluation of widths in the resist structures. Imperfections in the open stencil mask give rise to the main part of edge roughness, especially if electrostatic charging occurs. Lines and spaces in PMMA resist developed with high contrast show a roughness of 10 nm per edge if the mask has no defects and the projection system and resist performance have been adjusted satisfactorily. Statistical fluctuations of the ion current have no influence if a medium sensitive resist like PMMA is used resulting in exposure times of 100 ms.